Low noise thermistor assembly and method



June 24, 1969 M. SAPOFF ETAL 3,452,314

LOW NOISE THERMISTOR ASSEMBLY AND METHOD Filed May 22. 1967 FIG. 2 315FIG. 4

FIG. 5

United States Patent 01 lice 3,452,3'14 Patented June 24, 1969 3,452,314LOW NOISE THERMISTOR ASSEMBLY AND METHOD Meyer Sapotr, West Orange, andJohn G. Froemel, Verona, N.J., assignors to Victory EngineeringCorporation, Springfield, N.J., a corporation of elaware Filed May 22,1967, Ser. No. 640,200 Int. Cl. H01c 7/04, 7/00 U.S. Cl. 338-22 4 ClaimsABSTRACT OF THE DISCLOSURE Thermistor materials are deposited on asubstrate by evaporation or sputtering. One or more terminals and aresistance pattern of gold is then deposited through a mask on desiredareas of the thermistor material. The thermistor assembly is then heatedin a furnace at 500 degrees centigrade for fifteen minutes to partiallydiifuse the gold into the thermistor material. After the heating cycle,platinum wires iridium) are welded to the gold terminals. The resultingthermistors inject considerably less noise into a circuit when passingcurrent.

Background of the invention This invention relates to a means and methodfor making thermistors producing almost no noise when passing current.The invention has particular relationship to thermistors which are to beused in amplifier or telephone circuits passing voice frequencycurrents.

Summary of the invention Therrnistors used for measuring purposes,particularly in bridges, may inject noises into the circuit withoutproducing any harmful effects. However, when used in any circuit whichis connected to a sound transducer it is important to keep the noise aslow as possible. It has been found that the main source of noise is inthe connection between the thermistor material and the metal electrodeswhich make contact with them. Plating or eavporating a metal onto athermistor surface is always susceptible to microscopic arcing and minorcircuit breaking due to insufficient pressur ebetween the two materials.It has been found that the use of gold sputtered on a terminal area andthe surface of the thermistor and then treated in a furnace will eitheralloy or diifuse into the thermistor material and produce a bond that issubstantially free of noise. The thermistor material may be evaporatedonto a nonconducting substrate or it may be deposited onto the substrateby sputtering techniques in a rarified atmosphere of an inert gas.

Brief description of figures FIGURE 1 is a plan view of an area ofthermistor material deposited evenly onto a nonconducting substrate.

FIGURE 2 is a plan view of one form of the invention and shows thethermistor material deposited in a resistor pattern on a substrate.

FIGURE 3 is a plan view of the device shown in FIGURE 1, with the goldterminal material deposited on the thermistor material in anotherpattern which will produce a low resistance between the terminals. Thisview also shows the terminal wires.

FIGURE 4 is a cross-sectional view showing the substrate, the thermistormaterial, and a gold terminal before the application of the heatingcycle.

FIGURE 5 is a cross-sectional view taken along line 5-5 of FIGURE 2 andis similar to FIGURE 4 except that the gold has been alloyed into thethermistor material and a terminal wire has been welded to the gold.

Description 0] preferred embodiment Referring now to FIGURE 1, asubstrate 10 is shown with an area of thermistor material 11 evaporatedthereon. The deposition of the thermistor material may be done by eitherevaporation or sputtering. This constitutes the first step in theproduction of a low resistance thermistor. Next, two gold terminals 12,13, are deposited on the thermistor material as shown in FIGURE 3. Thegold is preferably deposited by sputtering through a stainless steelmask which can be positioned directly in front of the thermistormaterial during the sputtering process. Apparatus for performing thisstep is described in co-pending patent application Ser. No. 435,119,filed Feb. 25, 1965, in the names of John G. Fromel and Meyer Sapoff,for Thin Film Resistance Elements and Method, now abandoned.

The thermistor array is next placed in a furnace and heated to 500centigrade for about fifteen minutes. This heating does not change thethermistor material because it is made up of a mixture of oxides butcauses the gold to diffuse partially into the thermistor. After heating,Wires 14 and 15 are welded to terminal areas 16- and 17, and thethermistor is ready for use.

The above described thermistor is obviously a low resistance elementsince the distances between terminals is short and since the terminalsare long. If a high resistance thermistor is desired, the type shown inFIGURE 2 is fabricated. In this case the thermistor material 20 issputtered onto a base 10 through a mask so that the thermistor materialhas a long path between its end portions '21 and 22. In order to providesuitable terminals for inclusion in a circuit, gold areas 23 and 24 aredeposited through a mask in a manner similar to that described above.The thermistor is next given a heat treatment as described above byheating to 500 centigrade for fifteen minutes. After the heatingoperation, wire terminals 14 and 15 are added.

FIGURES 4 and 5 illustrate the method of noise re duction due toheating. In FIGURE 4, the substrate 10 carries a layer of thermistormaterial 22 and a gold terminal 24. As is shown in the drawing, the goldrests on top of the thermistor material but there is no evidence ofcombination. After the heating cycle, the thermistor material 22 andgold terminal 24 have been diffused together as is indicated by themerging cross-hatching lines in FIGURE 5. This construction accounts forthe lack of noise. A platinum wire 15 or any other type of wire may bewelded or soldered to the gold terminal 24 and the thermistor is readyfor use.

It will be obvious from the above description that many other forms ofthermistor material and many other arrangements of gold terminals can beused for this process.

Having thus fully described the invention what is claimed as new anddesired to be secured by Letters Patent of the United States is:

1. A thermistor comprising a non-conductive substrate, thermistormaterial deposited on the substrate to form a resistance, a plurality ofgold terminal areas deposited on the thermistor material atpredetermined positions to form terminals for the application of anelectric voltage, said gold terminals diffused into the thermistormaterial by the application of heat after the gold has been deposited,and a terminal wire secured to each of said gold terminals forconnection to an external circuit.

2. A thermistor as claimed in claim 1 wherein the thermistor material isa mixture of metal oxides.

3. A method of forming thermistors having gold diffused terminals forthe reduction of electric circuit noise comprising the following steps:depositing thermistor materia1 on a non-conductive substrate, depositinggold on predetermined terminal areas to jorm circuit terminals, heatingthe array in a furnace of a temperature within the range of 400 to 600degrees centigrade for a time interval in excess of 10 minutes, andfinally attaching wire terminals to the gold terminals to form anelectric circuit component.

4. A method according to claim 3 wherein said gold is sputtered from atarget in a rarified atmosphere of inert gas through a mask to depositonly on predetermined areas.

REUBEN EPSTEIN,

References Cited UNITED STATES PATENTS 6/1947 Pearson 33822 8/ 1956Eisler 338-309 10/1956 Thomsen 338-309 1/1959 Howatt 29612 12/1960Baasch 338-22 11/1965 Girard 3-3 822 9/ 1966 Smith 29-612 PrimaryExaminer.

US. Cl. X.R.

